public class MyQueue {
    static class ListNode{
        public int val;
        public ListNode prev;
        public ListNode next;

        public ListNode(int val) {
            this.val = val;
        }
    }
    public ListNode first=null;
    public ListNode last=null;
    public int usedSize=0;
    public void offer(int val){
        ListNode node=new ListNode(val);
        if (isEmpty()){
            first=last=node;
        }else{
            node.prev=last;
            last.next=node;
            last=last.next;
            usedSize++;
        }

    }
    //删除头结点
    public int poll(){
        if (first==null){
            return -1;
        }
        int val= first.val;
        first=first.next;
        if (first!=null){
            first.prev=null;
        }
        usedSize--;
        return val;
    }
    public int peek(){
        if (first==null){
            return -1;
        }
        int val= first.val;
        return val;
    }
    public boolean isEmpty(){
        return usedSize==0;
    }
}






















